Need suspension help

Hi, new to posting on the site.Need help with suspension .My question is what causes the rear tyre on a 08 gen 11 to scrub in the centre while riding on a normal freeway at average speed of 160km/h.The tyre is a BT56 , suspension setting is rebound out 12 clicks, compression 8 clicks, spring has 7mm thread showing above,tyre pressure is 2.8bars, rider weight is 115 kgs.Sag rear is 32mm with gear.Scrubbing only occurs while riding on the freeway and not in town.Would really appreciate the help unfortunately no pictures avail.I need to sought out problem before fitting new tyre, existing tyre only 1700km with average riding thanx.